Thermodynamic Study of the Complexation of Trivalent Actinide and Lanthanide Cations by ADPTZ, a Tridentate N-Donor Ligand

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Abstract
To better understand the bonding in complexes of f-elements by polydentate N-donor ligands, the complexation of americium(III) and lanthanide(III) cations by 2-amino-4,6-di-(pyridin-2-yl)-1,3,5-triazine (ADPTZ) was studied using a thermodynamic approach. The stability constants of the 1:1 complexes in a methanol/water mixture (75/25 vol %) were determined by UV−visible spectrophotometry for every lanthanide(III) ion (except promethium), and yttrium(III) and americium(III) cations. The thermodynamic parameters (ΔH°, ΔS°) of complexation were determined from the temperature dependence of the stability constants and by microcalorimetry. The trends of the variations of ΔG°, ΔH°, and ΔS° across the lanthanide series are compared with published results for other tridentate ligands and confirm strongly ionic bonding in the lanthanide−ADPTZ complexes. Comparison of the thermodynamic properties between the Am− and Ln−ADPTZ complexes highlights an increase in stability of the complexes by a factor of 20 in favor of the americium cation. This difference arises from a more exothermic reaction enthalpy in the case of Am, which is correlated with a greater degree of covalency in the americium−nitrogen bonds. Quantum chemistry calculations performed on a series of trivalent actinide and lanthanide−ADPTZ complexes support the experimental results, showing a slightly greater covalence in the actinide−ligand bonds that originates from a charge transfer from the ligand σ orbitals to the 5f and 6d orbitals of the actinide ion.

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